Some people can be knocked in the head and simply shrug it off.
Others get an elbow to the jaw — such as Kane Barrett — and that can be their career all over.
Concussion is an injury which has no set diagnosis and no quick cure.
But one thing for sure — it is having a huge impact on the game.
A quick check of statistics shows an entire team of New Zealand professional players has been lost to the game because of concussion in the past decade.
All Blacks have given their livelihood away and top players have had their careers cut short and lucrative pay days lost due to concussion.
All Black Ben Smith has spoken about his thoughts of having to give up the sport last year because of the head knocks he received in 2017. One of them was due to an internal ear problem but the other two head knocks in a short period of time were concussion-related.
No doubt plenty of All Black and Highlanders fans had their hearts in their mouth last Friday night at Forsyth Barr Stadium when Smith came crashing down from a high take and landed on his back.
He got up all right but it looked ugly and the referee immediately stopped the game.
Concussion, described as a mild traumatic brain injury which temporarily affects brain functioning, has been in the game for as long as it has been played but has come into focus in the past five years as player welfare becomes more of an issue.
It can never be rid of in the game as it is a collision sport so concussions will occur. Although facts and anecdotes seem blurred in the concussion debate, most of them occur in tackles and most concussions affect forwards.
Last year, 17 All Blacks were affected by concussion, about a third of those who took to the field in the black jersey.
Former Otago midfield back Jayden Spence is back in his home town of Alexandra after having to give the sport away because of concussion.
He got a bad one in 2013 while playing club rugby in Dunedin and then picked up two in the past year while playing for Bordeaux in France.
He went to see two medical specialists in Paris after the second concussion and they left him with one option — retirement.
"It wasn’t really a tough decision. I had a bad one and missed about four to five months. Then I played another five to six games and got another one and decided to pull the pin.
"To keep playing was just not worth it ... I felt groggy for a while but I’m all good now.
"It was hard to walk away but when it comes down to it, it is your life and you’ve got your whole future in front of you."
The 26-year-old is not sure what the future holds although he and partner Kate Davidson are due to have a baby in August.
Spence, who played 49 games for Otago, said becoming a father probably played a small part in him giving the game away but he had a plenty of good times to look back on.
His contract was up at Bordeaux but he would have stayed on if the club was keen. But his concussion meant negotiations never went far.
The Concussed XV
New Zealand rugby players forced to give up the game because of concussion. —
Ben Afeaki: Got smacked in a game for the Chiefs at the start of 2014. The prop never came right and ended it more than a year later.
Craig Clarke: The captain and lock led the Chiefs to glory in 2012-13. Went to Ireland but was said to have had his 10th concussion in two years. Stopped playing and retired shortly afterwards.
Sean Polwart: A loose forward who got hit in a training ground incident in 2015 and retired two years later having never got back on the field.
Lee Allan: Loose forward from South Otago, who played more than 30 times for Otago and also played for the Highlanders. Injured in early 2015 and missed all that season. Came back in 2016 bu,t after another couple of knocks, he ended it at the end of that year.
Shane Christie: Came late to the professional game and played for Tasman and the Highlanders. The loose forward ws hit in a game in South Africa in 2016 and, although he played for Tasman again, he did not appear for the Highlanders in 2017 and retired last month.
Mark Reddish: An honest toiler who played out of his skin for the Highlanders in their march to the title in 2015. Went to England but the lock only played five games before he was sidelined and was forced to give the game away.
Jayden Spence: The Otago outside back suffered a nasty concussion in 2013. Got back on the field but two in France last season forced him to pull the pin.
Jared Payne: The former New Zealander went to Ireland and played at centre for his adopted country and the British and Irish Lions. Hit in the head in a tackle against the Chiefs on the Lions tour last year and never played again. Has moved into coaching.
James Broadhurst: Played one half for the All Blacks at lock in 2015, but got injured later that year in a provincial game and never played again.
Steve Devine: Was said to be on borrowed time as early as 2006 but the halfback kept playing for another couple of years before hanging up the boots.
Jason Eaton: The big lock had a long career, firstly in New Zealand and then overseas. However, he picked up a few head knocks and retired in France last month.
Reggie Goodes: Born in South Africa, Goodes was a hard-working prop who started in the first-class game early. Three serious concussions since 2014 forced him to retire this year.
Kane Barrett: Another Barrett brother who got an elbow to the jaw in 2014 while training with the Blues. The loose forward never played again. Now a stock buyer in Hawkes Bay.
Shane Cleaver: The prop picked up a knock for the Chiefs in 2012 and then three in a short period of time and gave it away in 2013.
Leon MacDonald: He seemed to have a run of head knocks right throughout his career. Went to Japan for a period but another knock led there ended his playing days in 2010.